1. Field of the Invention
The invention relates to a semiconductor device with current limiting properties, and more particularly (although not exclusively) to a transistor or a vertical-cavity surface-emitting laser (VCSEL).
2. Discussion of Prior Art
Thermal runaway has been a known problem in semiconductor devices since the 1950s. It is particularly relevant to devices composed of narrow bandgap materials operated at ambient temperature and those of wider bandgap materials operated at elevated temperatures (eg high power devices). It results from device resistance falling with an increase in temperature, device current increasing in consequence, temperature increasing and resistance falling further and so on. It can lead to catastrophic device failure.
One form of semiconductor device incorporates an electrically parallel array of current-carrying elements intended to carry like currents, and in this device small growth or fabrication non-uniformities amongst individual elements can lead to localised heating during operation. This results in an individual element of the device carrying most of the current through the device by the process of thermal runaway. Examples of such semiconductor devices are multi-finger bipolar power transistors and VCSEL arrays. If a localised defect should induce an increase of current in a particular part of the device, an increase in temperature will be produced which in turn induces further increments in the localised current by increasing the number of charge carriers. Positive electrical and thermal feedback in a locality results in a non-uniform current distribution amongst the various elements, causing failure of the device as a whole, and may also lead to the destruction of the individual element concerned. The effect whereby a single element conducts most of the current through a device is known as xe2x80x9ccurrent-hoggingxe2x80x9d.
In the case of an array of electrically-parallel VCSELs, the current-hogging problem may lead to catastrophic damage in individual lasers which have a lower resistance than the other members of the array. Variations in the resistances of these lasers also result in variations in the currents conducted by them, and hence to non-uniform brightness across the array.
In the context of bipolar transistors, the current-hogging problem has meant that multi-finger devices have been unreliable during high-power operation in the absence of stabilisation schemes to eliminate the problem of thermal runaway. Current-hogging by a single emitter finger causes failure of the entire transistor, i. e. a collapse in current gain, output power, and available voltage swing during radio-frequency (RF) operation. Permanent damage may occur to the current-hogging emitter finger.
Several current stabilisation schemes for multi-finger power transistors have been proposed. One such scheme involves the integration of a fixed stabilising resistor, or xe2x80x9cballast resistorxe2x80x9d, into the structure in order to limit the current flowing through any single finger. Fixed ballast resistors have been integrated into the emitter and base regions of transistors, as reported by Gao et al and Liu et at in the IEEE Transactions on Electron Devices, Volume 21, No. 7, 1991 and Volume 38, No. 2, 1996 respectively. However such designs have the disadvantage that under low current and/or temperature conditions, there is a significant penalty to the RF power efficiency of the transistor because current is limited through all the fingers of the device simultaneously, including those fingers which do not have the problem of thermal runaway.
In another stabilisation scheme, known as thermal shunting, the emitter fingers of a transistor are thermally coupled to each other by a metal bridge in order to reduce temperature non-uniformities. In IEEE Electron Device Letters, Volume 17, No. 1, 1996, an example of a thermally-shunted bipolar transistor was disclosed by Sewell et al. In yet another scheme, disclosed at the 1994 IEEE International Electron Devices Meeting by Yang et al, a recess is etched into the reverse side of the substrate opposite the active layers, and metallised to form a heatsink. Schemes such as these involve many processing steps in addition to wafer growth and processing, significantly increasing the overall processing time and cost.
It is an object of the invention to provide an alternative form of semiconductor device in which current is limited.
The present invention provides a transistor comprising an emitter incorporating a superlattice structure, a base and a collector characterised in that the transistor has an emitter-current versus base-emitter voltage characteristic which exhibits a first region below a critical emitter-current and having a non-zero slope of average value m1, a reduction in slope from the value m1 at the critical emitter-current; and a current plateau above the critical emitter-current, the plateau having an average slope m2 in the range xe2x88x920.05 m1xe2x89xa6m2xe2x89xa6+0.05 m1.
The invention provides the advantage that a current-carrying element within a semiconductor device of the invention is protected from thermal runaway. Also, in a semiconductor device of the invention having a plurality of electrically parallel current carrying elements (for example a multi-finger transistor or a VCSEL array) the invention provides the advantage that any current-hogging by one or more elements of the device, and hence imbalance in current distribution within the device, is counteracted . This is achieved by increases in the resistances of those elements which begin to draw abnormally high currents without changing the resistances of, or currents in, the remaining elements. Unnecessary reductions in the currents carried by the remaining elements are therefore avoided and the functioning of these elements is unaffected. The risk of catastrophic damage to an element which begins to draw an abnormally high current is reduced.
A current-carrying element of a device of the invention has a non-linear current versus voltage characteristic. More specifically, a current-carrying element of a device of the invention has a differential resistance which is substantially constant up to a critical current, and then increases very rapidly with increasing current to a much greater value. Current-carrying elements of devices of the invention are therefore protected against thermal runaway.
Devices of the invention may be entirely fabricated using epitaxial growth methods and processing methods familiar to those skilled in the art of semiconductor device fabrication.
A device of the invention has a further advantage over a prior art device incorporating a fixed ballast resistor. The superlattice structure is typically ten times thinner than such a resistor, providing a shorter heat-conducting path from the active region of the semiconductor device to the contact metallisation. The resulting improvement in heat dissipation increases the lifetime of the device and reduces its susceptibility to thermal runaway.
Devices of the invention in the form of bipolar transistors may be used in radio frequency (RF) amplifying circuits. They can provide a current limit within the natural current limit of the transistor, and hence current-clipping of the input waveform, which makes it possible to achieve very high efficiency RF amplification. Presently, current-clipping is achieved in RF amplifiers by driving the transistor to its natural saturation current, resulting in charge storage in the transistor which severely degrades the gain bandwidth of the amplifier.
A semiconductor device of the invention may be in the form of a heterojunction bipolar transistor device in which the superlattice structure adjoins a first active region and forms an emitter in combination with the first active region and a second active region comprises both the base and collector, the second active region adjoining the first active region at a side thereof opposite to that adjoining the superlattice structure.
A semiconductor device of the invention may be in the form of a semiconductor laser having first and second active regions, the first active region including an optical gain layer and the superlattice structure adjoining the first active region at a side thereof opposite to that adjoining the second active region. The optical gain layer may have optical cladding. The device may be replicated to form an array of lasers. It may be constructed at least partly of layers of the AlxGa1xe2x88x92xAs material system, where 0xe2x89xa6xxe2x89xa61.
A semiconductor device of the invention may have a plurality of individual first active regions all adjoining a second active region, and the superlattice structure may comprise individual superlattice structures each adjoining a respective first active region and each having a current versus voltage characteristic which exhibits a change in resistance at a critical current to prevent current within a corresponding first active region from reaching an undesirable level. It may be a heterojunction bipolar transistor device having base, collector and emitter elements, each superlattice structure forming an emitter element in combination with a respective first active region, the second active region comprising both base and collector elements and each superlattice structure adjoining a respective first active region at a side thereof opposite to that adjoining the second active region. The transistor base element may be of InGaP and other device parts may be of the AlxGa1xe2x88x92xAs material system, where x=0, 0.15 or 0.33.